The invention pertains to a process and to a device for reducing the nitrous oxide which is formed during the catalytic combustion of ammonia and oxygen to form nitrogen oxides, where a catalyst system consisting of at least one first catalyst mesh element and at least a second catalyst mesh element is used for the catalytic combustion of ammonia and oxygen to form nitrogen oxides, and where at least one first catalyst mesh element is a platinum-rhodium mesh element.
A familiar problem in the production of nitric acid by the combustion of ammonia with oxygen to form suitable nitrogen oxides is the fact that nitrous oxide (laughing gas) is also formed, which is under suspicion of contributing to the destruction of the earth""s ozone layer. In the production of nitric acid, therefore, it is extremely important to effectively destroy the N2O which has formed or to prevent nitrous oxide from being formed as completely as possible in the first place. The attempt to achieve these ends should not impair the yield of the desired nitrogen oxides.
DE 198-19,882 A1 discloses a process for the catalytic decomposition of the N2O present in a gas mixture obtained during the production of nitric acid by the catalytic oxidation of ammonia. This process makes use of a catalyst for the decomposition of N2O and is characterized in that the hot gas mixture obtained from the catalytic oxidation of ammonia is brought into contact with the N2O-decomposing catalyst before the mixture is cooled.
DE-OS 2,239,514 discloses an arrangement of catalyst gauze strips for accelerating the reaction between two or more gases flowing through the gauze. On the downward-directed flow side of a group of gauze strips of precious metal material, a second group of gauze strips consisting of non-precious metal material, relatively difficult-to-vaporize metal material, and metal material which can be carried away by the gases is provided. The second group supports the acceleration of the reaction during the operation of the system.
DE-OS 1,959,137 discloses a catalyst with a reduced platinum and rhodium content of 12-20 wt. % for the oxidation of ammonia to nitrogen oxide.
In Hollemann-Wiberg, Lehrbuch der anorganischen Chemie [Textbook of Organic Chemistry], 71-b 80th edition, Verlag Walter de Gruyter and Co., Berlin, 1971, p. 360, an ammonia combustion element for recovering nitrogen oxide with a platinum mesh catalyst and larger systems with several stacked wire mesh elements per combustion element are described.
EP 0,359,286 B1 describes a process for reducing the nitrous oxide which is formed during the catalytic combustion of ammonia and oxygen to form nitrogen oxides. The nitrous oxide is cooled in a heat recovery unit and then absorbed in water and/or dilute nitric acid. The process is characterized in that the hot combustion gases are given a retention time of 0.1-3 seconds before they are cooled. In this way, it is possible to decompose up to 90% of the N2O which has formed.
EP 0,611,041 B1 discloses a process for reducing the N2O emissions during the startup phase of an ammonia oxidation reaction. A catalyst mesh based on platinum is used together with a platinum collector mesh element, which comprises fibers of a palladium alloy, said palladium alloy containing 0.1-5 wt. % of cobalt.
DE 198-05,202 A1 describes a process for the production of nitric acid in which ammonia is burned on at least one catalyst mesh element, especially on a platinum mesh element, as oxygen is being supplied. The reaction gases are then cooled. Before the reaction gases are cooled downstream of the catalyst mesh, they are conducted over a heat-resistant catalyst to convert the N2O contained in the reaction gases.
The processes indicated above either have unsatisfactory N2O decomposition rates or require a very complicated apparatus, which must be considered disadvantageous in an economic sense.
The problem is therefore to find a novel method and a novel device for reducing nitrous oxide while avoiding at least some of the disadvantages listed above and especially to provide an efficient process and a corresponding device which are low in cost in terms of the required apparatus.
Both in the process according to the invention and in the corresponding device according to the invention, ammonia is mixed with oxygen and burned at a temperature of approximately 860xc2x0 C by passing it quickly over a first catalyst mesh element, which consists of platinum-rhodium mesh typically containing 5.0-10.0 wt. % of rhodium, and then by passing it quickly over a second catalyst mesh element, this second catalyst consisting of palladium-rhodium mesh with at least 92 wt. % (or more) of palladium, 2-4 wt. % of rhodium, and the remainder of platinum or alternatively of 82-83 wt. % of palladium, 2.5-3.5 wt. % of rhodium, and the remainder of platinum, as a result of which the proportion of nitrous oxide is reduced in a highly efficient manner. The mesh used here is a gas-permeable structure which is essentially metallic and which is produced by, for example, knotting, knitting, weaving, or the random laying of fibers.
The surprising result was obtained that, through the simultaneous use of the two catalyst mesh elements according to the invention, the proportion of N2O that forms could be decreased significantly right from the start, and simultaneously NO could be produced in a highly efficient manner.
It is advantageous, first, for the palladium-rhodium mesh to have a rhodium content of 2.5-3 wt. % and a palladium content of 82.5%, because in this way an especially low level of N2O formation can be reached.
It is also advantageous for the palladium-rhodium mesh element to be separated from the platinum-rhodium mesh element by least one third mesh element, so that in this way it is possible to prevent the different types of mesh elements from becoming welded together. As material for the third mesh element, a heat-resisting steel (FeCrAl alloy, e.g., Megapyr or Kanthal) has been found to give good results.
Finally, it can be advantageous for the palladium-rhodium mesh to contain an additional metal from the group consisting of iridium, ruthenium, rhenium, cobalt, nickel, copper, and gold to increase the mechanical stability of the palladium-rhodium catalyst mesh elements without interfering with their catalytic function.